Crystalline phase transition in as-synthesized pure silica zeolite RTH containing tetra-alkyl phosphonium as organic structure directing agent

Abstract

The choice of structure directing agents (SDAs) in zeolite synthesis significantly impacts the arrangement of active sites, thereby influencing the stabilization of reaction intermediates with profound implications for catalytic applications. Therefore, understanding the distribution of SDAs along with the substitution of heteroatoms for silicon in zeolites is imperative for tailoring optimized materials for specific applications. This study is centered on the synthesis of all-silica RTH type zeolites in the presence of fluoride, utilizing triisopropyl(methyl)phosphonium as the organic SDA (OSDA). Zeolites produced under varying conditions of time and temperature exhibit differences in their X-ray diffractograms, indicating the presence of two distinct crystalline phases. The ¹⁹F NMR spectra confirm the presence of fluoride within the small rth cage and exhibit two distinct signals depending on the sample. The ²⁹Si NMR spectra reveal the existence of penta-coordinated F-SiO₄ species, resulting in sixteen non-equivalent Si sites. Through ab initio DFT methods, the stabilization energy and ²⁹Si chemical shielding of several models featuring F-SiO₄ situated at all crystallographic sites were computed. Comparison with experimental results enabled the identification of the framework position where the five-coordinate silicon is located, which differs between the two crystalline phases of the as-synthesized RTH zeolites. Consequently, the placement of fluoride in either of these two sites within the RTH zeolite can be controlled during the synthesis. It is expected that this methodology can be extended to manipulate the position of trivalent atoms (e.g., Al³⁺ or B³⁺), which can affect the catalytic properties of the RTH zeolite.